ABSTRACT - OVERALL More than 1,850 elderly or medically infirm patients with advanced hematologic malignancies have received HLA-matched related or unrelated or HLA-haploidentical hematopoietic cell transplantation (HCT) on reduced- intensity conditioning regimens that were translated from canine studies under this grant. While overall 5-year survivals were encouraging, we identified two problems that accounted for nearly all of the treatment failures: non-relapse mortality (NRM) from graft-versus-host-disease (GVHD)-related causes and relapse-related mortality. Moreover, acute GVHD did not convey GVT effects. In contrast, chronic GVHD showed significant GVT effects; however, this benefit was offset by increased NRM. These findings set the theme for the current grant. We propose three projects, one preclinical and two clinical, which focus on reducing GVHD-related NRM and relapse mortality. The theme of the preclinical Project 1 is to minimize GVHD-related NRM. We will use a DLA-mismatched canine model that has served to develop nearly all of our GVHD prevention and treatment used clinically. Aim 1 will focus on preventing acute GVHD, and Aim 2 proposes new treatment strategies for chronic GVHD. Developing GVHD is consistent with T-cell activation despite standard immunosuppression. We have generated or identified monoclonal antibodies (mAbs) specific for canine T-cell regulatory molecules. Guided by the results of linked mechanistic studies, we will use the mAbs to block T-cell costimulation and/or downregulate or eliminate activated T-cells. We hypothesize that the current high incidence of acute GVHD can be reduced and that chronic GVHD can be treated more effectively, reducing both the duration of the current long-term immunosuppressive therapy (median 2.5 years) for transplanted patients and the risk of fatal infections. The clinical Projects 2 and 3 address relapse in patients with advanced acute leukemias and myelodysplasias (Project 2) and B-cell malignancies (Project 3) as well as extending allogeneic HCT to include patients who lack HLA-matched donors. Both projects propose dose-escalation studies for HLA-matched HCT recipients using an anti-CD45 mAb coupled to an alpha-emitting radionuclide, astatine-211 (211At), in addition to the standard fludarabine (FLU)/2Gy total body irradiation (TBI) conditioning regimen. This novel approach is based on extensive preclinical studies in our canine model. We anticipate a significant reduction in pretransplant tumor burden from the addition of the 211At-labeled mAb and, thus, a corresponding reduction in relapse risk after HCT. Both projects will also address the relapse problem in HLA-haploidentical recipients. Project 2 proposes dose-escalation studies with 211At-labeled anti-CD45 mAb in addition to FLU/cyclophosphamide/2Gy TBI conditioning. Project 3 proposes a study of natural killer cell infusions from the HLA-haploidentical donors after reduced-intensity conditioning. A concurrent trial after myeloablative conditioning will study augmentation of HLA-haploidentical HCT with gene-modified T-cells.